Separation of cyclohexane from methoylcyclopentane by crystallization



March 8, 1955 M. R. CINES 2,703,818

SEPARATION OF CYCLOHEXANE FROM METHYLCYCLOPENTANE BY CRYSTALLIZATIONFiled Jan. 4, 1949 2 Sheets-Sheet 2 7 Y 4 v 7 A y lo FRACTIONATORCRYSTALLIZER FEED f FIG. 2

INVENTOR.

M. R. CINES BY W nrrnDA/FYS v tane.

United States Patent Ofiice a 2,703,818 SEPARATION OF CYCIDHEXANE FROMMETH- YLCYCIJOPENTANE BY CRYSTALLIZATION,

.This invention relates to,th e separation of hydrocarbons.

by crystallization. A specific aspect of the invention relates to amethod of resolving certain binary hydrocarbon mixtures that form solidsolutions.-

It is already known to resolve mixtures of hydrocarbons or other organiccompounds by fractional crystallization. In such processes, the mixturein question is cooled until crystals form and the crystals are separatedfrom the supernatant liquid. It is customary m such processes tomaintain the crystallization temperature not much lower thanthatrequired for the formation of appreciable quantities of crystals. 1

I have found that the above method is not .umversally applicable to theresolution of binary mixtures of hydrocarbons. It has recently beenreported by Tooke and Aston, JACS 67-2275 (1945), that certain pairs ofhydrocarbons probably form solid solutions. Hydrocarbon pairsspecifically mentioned are: 2,3-dimethylbutane and 2,2-dimethylbutane;2,3-dimethylbutane and 2-methylpentane; cyclopentane and2,2-dimethylbutane; 1,2-butadiene and cis-butene-Z; cyclohexane andmethylcyclo pentane; and 2,2,3-trimethylbutane and 2,4-dimethylpen- Theformation of solid solutions in such a system severely complicates theseparation of pure components. I have observed that a number ofhydrocarbons that enter into the formation of solid solutions exist inmore than one crystal form.

The invention has several objects, viz.,

To provide a process for separating a hydrocarbon from a mixture ofhydrocarbons whereby the hydrocarbon is recovered in pure form.

To provide a relatively simple process for resolving a mixture ofhydrocarbons so as to obtain the hydrocarbon constituents in relativelypure form.

To provide for the separation in pure form of a hydrocarbon from abinary mixture of hydrocarbons, which hydrocarbon forms, solid solutionwith the other hydr'ocarbon in the mixture.

Other objects of the invention will become apparent from theaccompanying disclosure.

The following discussion will concern mainly binary mixtures consistingof cyclohexane and methylcyclopentane, although the invention in itsbroadest scope is not limited to these specific mixtures. The invention,in the broadest sense, applies to mixtures, one component of which canexist in more than one crystal form and can form solid solutions with atleast one other component; one limitation is that one of said crystalforms, namelythe lower temperature stable one, be substantiallyincapable of forming solid solutions with said other component. Anotherlimitation is that the eutectic tem rature is below that for thesolid-phase transition 0 the one component.

In the drawings, Figure 1 represents a phase-equilibrium diagram for thesystem cyclohexane-methylcyclopentane; and Figure 2 is a flow diagramfor purification of hydrocarbons by crystallization according to theinvention.

Referring to Figure l, in which temperature is plotted againstcomposition, the area above the curves ADEF represents conditions underwhich a. single homogeneous liquid phase exists. Under conditionsrepresented by the area ACD, liquid exists in equilibrium with a solidsolution of methylcyclopentane in cyclohexane. Under conditionsrepresented by the area ABC the system consists of a single homogeneoussolid phase, which is a solution of methylcyclopentane in cyclohexane.In this solid solution, the cyclohexane exists in one crystalline formknown as the beta form. Under conditions repre- 2,703,81 Patented Mar.8, l9

difierent solid phases One phase is the solid solutionmethylcyclopentane in beta cyclohexane discussed abc The othersohd-phaseis cyclohexane in a difierent crysi lme form, known as the alpha form.Under conditit represented by the area K DEH, a solid phase existsequilibrium with a liquid phase. The liquid phase i mixture ofcyclohexane and methylcyclopentane z the solid phase is alphacyclohexane. Under conditi represented by the area EFG, --1iquid mixtureexists equilibrium with crystals of methylcyclopentane. In area belowthe line HEG, eutectic (an intimate r chamcal mixture of a lphacyclohexane and methylcyt pe ntane of composition 'ven by point E)exists. this area and to the left 0 the line B], the eutectic ex n admxture w th alpha cyclohexane, and on the rig lll admixture withmethylcyclopentane.

Whrle the line CD is shown on the phase diagram Figure l as representingapproximately C., point at which the change from liquid plus solid solutto alpha cyclohexane plus liquid methylcyclopent takes place lies in theregion between 100 C. approximately -1l0 C. The difliculties encounte mfixing this temperature at a specific point have b practicallyinsurmountable. A few of the difficul involved are the efiect ofsuper-cooling, slow equilibrii low temperature heat efiects withattendant difliculty determining temperature in this range, etc. Howefor practical operation, cooling to a temperature -1 l0 C. is considerednecessary in order to effect desired separation of pure alphacyclohexane from m ylcyclopentane.

It will be seen from studying this diagram that if i has a mixture of 70mol percent methylcyclopentane 30 mol percent cyclohexane and coolsit-to approxima l16 C., crystals of alpha cyclohexane separate can bewithdrawn as purified cyclohexane. If, howe one has a mixture of 30%methylcyclopentane and 7 cyclohexane and attempts to resolve it bycrystallizat crystals begin to separate at -40 C., and these crysconsist of a solid solution of methylcyclopentane in 'l cyclohexane. Oneskilled in the art and not having disclosure available would probablconclude that mixture could not be directly reso ved except by f tionalcrystallization. I have found, however, as Sill in Figure 1. that if themixture of 30% methylcyclo tane and 70% cyclohexane is cooled, not justto -40 but to -100 C. or preferably somewhat lower, nan 1 10 C., nosolid solution exists and the crystals fon at those temperatures aresubstantially pure alpha cy hexane. It will also be noted from thediagram, howe that the mixture should not be cooled below 144 for belowthis temperature the alpha cyclohexane cry: will be contaminated withsolid methylcyclopentane. has thus been established that, for mixturesof met cyclopentane and cyclohexane containing less than a] 52 molpercent methylcyclopentane, a critical temp ture region, namely 1l0 to144" 0., exists, out of which resolution into pure components cannoteffected by simple fractional crystallization. The in tion will thus beseen to apply, in a specific embodin to mixtures of cyclohexane andmethylcyclopentane taining less than 52 mol percent methylcyclopentanecomprises a method of resolution which involves coc the mixture to atemperature in the range of about -I to 144 C. and recovering crystalsof alpha cyclohe: of high purity.

Cyclohexane at atmospheric pressure boils at app] mately 81 C.Methylcyclopentane at the same pres boils at approximately 72 C. It istherefore obi that a separation by fractional distillation can be inHowever, when it is desirable to recover either of two components in ahigh degree of purity, i. e., a1 97%, rather expensive and complicatedfractional d lation will be necessary. The present invention, hOWtprovides a method of obtaining cyclohexane in a high degree of purity ina relatively small numbe steps and with relatively simple apparatus.

Refening to Figure 1 again, with a mixture of c:

sented by the area BKC, the mixture consists of two hexane andmethylcyclopentane containing more mol percent rnethylcyclopentane, itis feasible to ohi pure rnethylcyclopentane crystals by cooling the temto a temperature included in the area EFG. It i be noted, however, thatthis area represents a very ill temperature range, above which nocrystals would formed and below which the methylcyclopentane stals wouldbe contaminated with alpha cyclohexane stals. Such a method thereforerequires extremely se temperature control, which is rather difficulhat hlow temperatures. The process of my invention 'iates this difficulty byproviding an initial or prelimiy fractional distillation step in which alarge propori of the rnethylcyclopentane is removed as an overd product.The kettle product from this fractional :illation is concentrated withrespect to cyclohexane, ich can be removed in a highly purified form bycrysization in which extremely precise temperature coni is unnecessary.In such a process, it is desirable :oncentrate the kettle product atleast to such a point t it contains not more than 88 mol percent, moreirably not more than about 80-85%, and if desired, more than 52 molpercent rnethylcyclopentane. A :able lower limit of rnethylcyclopentanein the mixture about A mixture containing less than about mol percent ofrnethylcyclopentane may be sub ected crystallization in accordance WlihthlS invention, and stals of alpha cyclohexane of high purity may be:ctly recovered. process of this type is shown diagrammatically in ure2. A fraction consisting of rnethylcyclopentane l cyclohexane is passedto column 4 through feed 5, and an appreciable proportion of themethylcycloitane is removed overhead through outlet 6 and recred as oneproduct of the process or sub ected to furr purification as desired. Thekettle product containfor example mol percent rnethylcyclopentane, issed to crystallizer 7 through line 8. In crystallizer 7, mixture iscooled to a temperature in the range of 10 to --l44 C., and highlypurified crystals of alpha lohexane are recovered and withdrawn throughoutlet within the scope of this invention, but may not be preferred,since the resultant mixture, concentrated with respect tornethylcyclopentane, must still be cooled to temperatures below 110 C.to effect crystallization, and the volume of material subjected to thiscooling will be increased.

Certain modifications of the invention will become apparent to thoseskilled in the art and the illustrative details disclosed are not to beconstrued as imposing unnecessary limitations on the invention.

I claim: 1. A process for separating cyclohexane from a mix- 2. Theprocess of claim 1 including the additional-steps of concentrating themother liquor with respect to cyclohexane and repeating the cooling stepso as to crystallize and recover additional cyclohexane.

3. A process for separating cyclohexane from a mixture consistingessentially of cyclohexane and methyls the main product of the process.The supernatant iid or mother liquor which comprises a mixture ofthylcyclopentane and cyclohexane, now appreciably icentrated withrespect to methylcyclopentane, 18 reled to fractionator 4 throughconduit 10. ractionator 4 may be a conventional fractional disitiOIlsystem, the main column of which need not have .eptionally great heightor number of theoretical plates. rstallizer 7 will comprise conventionalcrystallizing, rigerating and crystal-separating means well known theart.

Example 8 mixture of cyclohexane and rnethylcyclopentane, in iroximatelyequal molar proportions, is subjected to ctional distillation, and anoverhead fraction of subritially pure rnethylcyclopentane is recovered.The tle product consists of 20 mol percent methylcyclopene and 80%cyclohexane. This preliminary fractiona- 1 reduces materially the volumeof mixture subjected crystallization and thus achieves a substantialreduci in refrigeration costs. The 20% methylcyclopene-80% cyclohexanemixture is passed to a crystallizer l cooled to 110 C., at whichtemperature crystals alpha cyclohexane form. The supernatant liquid iniilibrium with these crystals at this temperature comses 65 mol percentrnethylcyclopentane and 35 mol cent cyclohexane. The crystals areseparated from mother liquor in a centrifuge at a constant temperae of110 C., and the liquid is recycled'to the preinary fractionaldistillation step. The alpha cycloiane crystals recovered have a puritysubstantially ater than 99%, which is considerably higher than isvainable by simple fractional distillation in a -plate nmercial column.If desired further purification of crystals can be obtained by meltingand repeating fractional crystallization in accordance with thisinition.

"t is evident from examination of Figure 1 that, in the cyclopentane inwhich the cyclohexane amounts to between 48 and 90 mol per cent of themixture, which comprises cooling the mixture to a temperature in therange of 110 to 144 C. so as to form a cyclohexane crystals andrecovering said crystals as a product of the process.

4. A process for separating cyclohexane from a mixture consistingessentially of rnethylcyclopentane and cyclohexane containing a majormolar proportion of methylcyclopentane, which comprises concentratingsaid mixture to between 48 and 90 mol percent cyclohexane by fractionaldistillation, recovering an overhead fraction consisting essentially ofrnethylcyclopentane, passing a kettle product containing between 48 and90 mol percent cyclohexane as a feed to a crystallization zone, coolingsaid feed to a temperature below 110 C. but above -144 C. so as to formcrystalline a cyclohexane, recovering the crystalline a cyclohexane as aproduct of the process, and recycling mother liquor from thecrystallization zone to the distillation step so as to increase thecyclohexane concentration thereof to more than 50 mol percent.

5. A process for separating cyclohexane from a mixture consistingessentially of rnethylcyclopentane and cyclohexane in which thernethylcyclopentane amounts to less than 52 mol percent of the mixture,which comprises increasing the concentration of rnethylcyclopentane insaid mixture to more than 52 mol percent, thereafter 65 cooling theresulting mixture to a temperature in the range of 110 to 144 C. so asto form a cyclohexane frystals, and recovering said crystals from themother iquor.

6. A process for separating cyclohexane from a mixture consistingessentially of rnethylcyclopentane and cyclohexane containing more than88 mol percent methylcyclopentane which comprises concentrating saidmixture to between 15 and 90 mol percent cyclohexane by fractionaldistillation, cooling the resulting concentrated mixture to atemperature below C. but above 144" C. so as to form crystalline acyclohexane and recovering the crystalline a cyclohexane as a product ofthe process.

7. A process for separating cyclohexane from a mixture consistingessentially of rnethylcyclopentane and cyclohexane containing more than88 mol percent methylcyclopentane which comprises concentrating saidmixture to between 48 and 90 mol percent cyclohexane by fractionaldistillation, cooling the resulting concentrated mixture to atemperature below 110 C. but above 14 4 75 C. so as to form crystallinem cyclohexane and recovering the crystalline a cyclohexane as a productof the process.

References Cited in the file of this patent Taylor: Treatise on PhysicalChemistry, vol. 1, pub.

by D. Van Nostrand, N. Y. (1925), pages 432-4.

Bruun et al.: Bureau of Standards, Jour. Res., vol. 7,

pages 607-12 (1931).

Smittenberg et al.: Journal Am. Chem. Soc., vol. 60,

pages 17-22 (1938).

Tooke et al.: Jour. Am. Chem. Soc., vol. 67, pages

1. A PROCESS FOR SEPARATING CYCHOHEXANE FROM A MIXTURE CONSISTINGESSENTIALLY OF METHYLCYCLOPENTANE AND CYCLOHEXANE, WHICH COMPRISESCONCENTRATING SAID MIXTURE TO A MAJOR MOLAR PROPORTION OF CYCLOHEXANE,COOLING THE RESULTING MIXTURE TO A TEMPERATURE BELOW -110* C. BUT ABOVE-144* C. SO AS TO FORM A CYCHLOHEXANE CRYSTALS OF HIGHER THAN 99% PURITYAND RECOVERING SAID CRYSTALS FROM THE MOTHER LIQUOR AS A PRODUCT OF THEPROCESS.